How to blur a single object, Layers dont exist anymore? (2.92)(Permanent interface changes confuse every new user)
I am by no means used to work much with the compositor, that said, I hope what I am going to show here is not too far from what others would do.
Object Index method:

Activate the Object Index in the View Layer Properties:

Make sure the object in question has its own Object Properties -&gt; Relations-&gt;Pass Index:

Render the image with F12 if only that one object needs special treatment, otherwise give each object that needs it its own Pass Index like in step 2.
Go in the Compositor and activate Use Nodes:

simple example of a cube and sphere, whereas the sphere has pass index 1 and cube has pass index 2

Using the ID Mask node you can get the mask for the specific object or objects(same pass index objects are in the same mask)
Multiplying the rendered image with the mask we obtain the separate object and adding a Blur node we can then blur the specific object:

This would be blurring a specific object

Cryptomatte method:

Activate View Layer Properties-&gt;Passes-&gt;Cryptomatte-&gt;Object:

With the object already having its own pass, you can then render and get new output sockets on the Render Layers node in the compositor.
Connecting those to a Cryptomatte node and later the Blur node you get this:

The Cryptomatte node needs to be set up first
With the Pick output used, you can easy use the eyedropper Add to select the wanted object and switching to the Matte output this will show the mask created.
Switching now to the Image output you get only the selected object from the entire render.
Routing this image through the Blur node will then result in the same as above Object Index method.

UPDATE (Compositing the blurred object back into the image)
While not 100% sure, I believe this should be the result asked for:

All the difference is an Alpha Over node